linux-kernel - Re: [RFC PATCH 0/3] Use pageblock_order for cma and alloc_contig

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Date:   Tue, 23 Nov 2021 18:32:37 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Zi Yan <ziy@...dia.com>
Cc:     David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        linuxppc-dev@...ts.ozlabs.org,
        virtualization@...ts.linux-foundation.org,
        iommu@...ts.linux-foundation.org
Subject: Re: [RFC PATCH 0/3] Use pageblock_order for cma and
 alloc_contig_range alignment.

On 11/23/21 17:35, Zi Yan wrote:
> On 19 Nov 2021, at 10:15, Zi Yan wrote:
>>>> From what my understanding, cma required alignment of
>>>> max(MAX_ORDER - 1, pageblock_order), because when MIGRATE_CMA was introduced,
>>>> __free_one_page() does not prevent merging two different pageblocks, when
>>>> MAX_ORDER - 1 > pageblock_order. But current __free_one_page() implementation
>>>> does prevent that.
>>>
>>> But it does prevent that only for isolated pageblock, not CMA, and yout
>>> patchset doesn't seem to expand that to CMA? Or am I missing something.
>>
>> Yeah, you are right. Originally, I thought preventing merging isolated pageblock
>> with other types of pageblocks is sufficient, since MIGRATE_CMA is always
>> converted from MIGRATE_ISOLATE. But that is not true. I will rework the code.
>> Thanks for pointing this out.
>>
> 
> I find that two pageblocks with different migratetypes, like MIGRATE_RECLAIMABLE
> and MIGRATE_MOVABLE can be merged into a single free page after I checked
> __free_one_page() in detail and printed pageblock information during buddy page
> merging.

Yes, that can happen.

I am not sure what consequence it will cause. Do you have any idea?

For MIGRATE_RECLAIMABLE or MIGRATE_MOVABLE or even MIGRATE_UNMOVABLE it's
absolutely fine. As long as these pageblocks are fully free (and they are if
it's a single free page spanning 2 pageblocks), they can be of any of these
type, as they can be reused as needed without causing fragmentation.

But in case of MIGRATE_CMA and MIGRATE_ISOLATE, uncontrolled merging would
break the specifics of those types. That's why the code is careful for
MIGRATE_ISOLATE, and MIGRATE_CMA was until now done in MAX_ORDER granularity.